Kidd blood group system, classification of human blood based on the presence of glycoproteins known as Kidd (Jk) antigens on the surfaces of red blood cells. The Kidd glycoprotein functions to maintain the osmotic stability of red blood cells by acting as a transporter of urea. Antibodies that bind to the Kidd proteins can cause delayed transfusion reactions and erythroblastosis fetalis.

The Kidd blood group system, discovered in 1951, consists of three known antigens, designated Jka, Jkb, and Jk3, all of which are encoded by a gene known as SLC14A1 (solute carrier family 14, member 1). The Jka antigen occurs in more than 90 percent of blacks, 75 percent of whites, and 70 percent of Asians. The Jkb antigen is found in about 75 percent of whites and Asians and about 50 percent of blacks. The Jk3 antigen occurs in nearly 100 percent of all populations, and thus, antibodies against Jk3 are rare. The absence of both Jka and Jkb antigens, designated phenotypically as Jk(a−b−), is very rare, though it is found in roughly 1 percent of Polynesians. The most common Kidd phenotype is Jk(a+b+), which occurs in about 50 percent of whites and Asians and about 40 percent of blacks.

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